The present invention relates to a recording sheet for use in fusion thermal transfer recording and a method for fabricating thereof. More specifically, the present invention relates to a fusion thermal transfer recording sheet excellent in bar code printing and recording properties under a hot and humid atmosphere, and can afford a sharp full-color printing with gradation, and a method for fabricating such sheet.
Thermal transfer recording method is roughly classified into of sublimation thermal transfer system and fusion thermal transfer system. In the sublimation thermal transfer system, a thermal transfer ink ribbon, which is composed of a color material layer and a medium supporting thereof, is heated to sublime or gasify a dye contained in the color material layer, and the sublimed or gasified dye is fixed into an image accepting recording sheet, to thereby produce a dye image. On the other hand in the fusion thermal transfer system, as shown in FIG. 1, an ink ribbon 1 composed of a thermal fusion ink layer 5 and a base 4 supporting thereof is press-contacted with a thermal transfer image accepting recording sheet 2 between a drum 8 and a heat source 3 such as a thermal head, in which the thermal fusion ink layer 5 is fused by heating through the heat source 3 as controlled with electric signals, and the fused ink is directly transferred to a thermal transfer image accepting recording sheet 2, to thereby produce an image.
In a general practice of thermal transfer recording according to the fusion thermal transfer system, a support 7 per se may be responsible for image acceptance; or polyester layer, epoxy layer or primer layer having a good adhesiveness with the ink 5, may be provided on the surface of such support 7. The fusion thermal transfer image accepting recording sheet is composed of pulp paper; opaque synthetic paper comprising a stretched film of propylene-base resin containing an inorganic fine powder; or synthetic paper composed of a transparent polyethylene terephthalate stretched film or transparent polyolefin-base resin film having thereon a pigment coating containing an inorganic fine powder and a binder to thereby enhance the whiteness and dyeing property.
It is generally understood that using a synthetic paper having therein a lot of pores is preferable to achieve a desirable strength, dimensional stability and adhesion with a printing head, when considering the post-printing properties (e.g., durability against copying, writing with a pencil, and storage) of the thermal transfer image accepting recording sheet after the thermal transfer recording, as disclosed in JP-A-60-245593 (the code xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-61-112693, JP-A-3-216386 and JP-A-5-305780. Such synthetic papers are stretched using a tenter at a temperature lower than the H; melting points of polyolefin-base resins composing such papers to thereby form the internal pores, in order to achieve desirable opacity, softness, adhesion with the printing head and paper feed/discharge property. JP-A-8-80684 and JP-A-9-76647 disclose a synthetic paper comprising a micro-porous support containing inorganic fine powder (colloidal calcium carbonate), and having a primer-treated surface.
In recent years, full color printing is also increasingly adopted in the fusion thermal transfer recording, and need for improved gradation has shifted the main stream of a method of achieving gradation from such that forming dots of a constant diameter into a variable dot system forming individual dots with varied diameters. As for the recording sheet intended for use in full color recording using a wide range of printing energy, it is necessary that dot shape produced with the fusion thermal transfer ink is precisely reproduced (dot reproducibility), a sufficient amount of the ink can be transferred, and a high recording density is obtainable. To fulfill such requirements, there is proposed a technique in which ethylene-vinyl acetate copolymer is coated on a support made of a synthetic paper (JP-A-7-68956). Such recording sheet, however, was disadvantageous in that causing softening of the coated resin component during the thermal transfer of the ink, which excessively raised adhesive strength between the ink ribbon and the surface of the recording sheet, and undesirably resulted in blocking or omission in the printing.
Moreover, among the foregoing fusion thermal transfer image accepting recording sheets, a synthetic paper treated with a primer which comprises an aqueous solution of a nitrogen-containing polymer compound primer was suffered from a problem that the primer per se may degrade the transfer property of the fused ink since the surface of such sheet is likely to adhere (or adsorb) atmospheric moisture, which was causative of line breakage or no ink transfer during bar code printing.
On the other hand, in full-color fusion thermal transfer printing, the surface of the recording sheet should have properties differed from those in bar code printing, since inks of various colors individually having different ink components have to be transferred and overlapped. In particular to obtain a high-definition image, a precise dot reproducibility over a wide range of printing energy is required for the sheet, where the dot reproducibility of the conventional recording sheet was not always desirable enough.
It is therefore an object of the present invention to provide a fusion thermal transfer recording sheet which can solve the foregoing problem and can exhibit excellent properties both in bar code printing and full-color printing.
That is, the present invention is aimed at providing a fusion thermal transfer recording sheet causing no print omission even when printed under a hot and humid atmosphere, high in transfer density, and desirable in ink adhesiveness in bar code printing. The present invention is also aimed at providing a fusion thermal transfer recording sheet capable of producing a high-definition image in full-color printing. The present invention is further aimed at providing a simple method for fabricating a fusion thermal transfer recording sheet having such properties.
The present inventors found out after extensive investigations for solving the foregoing problems that the objective of the present invention is attainable by a sheet having a surface layer made of a uniaxially stretched film containing an inorganic fine powder, the surface of which being modified by hydrophilic treatment, and a base layer made of a uniaxially stretched film, which led us to propose the present invention.
That is, the present invention provides a fusion thermal transfer recording sheet having a base layer (A) of a uniaxially stretched film containing 40 to 85 wt % of a thermoplastic resin and 60 to 15 wt % of an inorganic or organic fine powder; and a surface layer (B) of a uniaxially stretched film, provided on at least one surface of the base layer (A), containing 30 to 90 wt % of a thermoplastic resin and 70 to 10 wt % of an inorganic fine powder which has an average grain size equals to or smaller than that of the inorganic or organic fine powder contained in the base layer (A) and has a grain surface modified by hydrophilic treatment.
Preferred embodiments of the present invention are such that the thermoplastic resin contained in the base layer (A) or the surface layer (B) is a polyolefin-base resin; such that the polyolefin-base resin is at least one polymer selected from the group consisting of propylene homopolymer, propylene copolymer, ethylene homopolymer and ethylene copolymer; such that the inorganic or organic fine powder contained in the base layer (A) has an average grain size of 0.6 to 3 xcexcm, and the inorganic fine powder contained in the surface layer (B) has an average grain size of 0.4 to 1.5 xcexcm; such that the base layer (A) or the surface layer (B) contains an inorganic fine powder selected from the group consisting of heavy calcium carbonate, clay and diatom earth; such that the surface layer (B) contains the inorganic fine powder modified by the hydrophilic treatment using an anionic polymer dispersant or a cationic polymer dispersant; such that the surface layer (B) contains heavy calcium carbonate powder modified by the hydrophilic treatment with an anionic polymer dispersant; such that the organic fine powder contained in the base layer (A) is selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, polyamide, polycarbonate, polyethylene naphthalate, polystyrene, melamine resin, polyethylene sulfite, polyimide, polyethyl ether ketone and polyphenylene sulfite; such that the organic fine powder contained in the base layer (A) has a melting point higher than that of the thermoplastic resin contained in the base layer (A) and incompatible therewith; such that a recording plane of the surface layer (B) has a smoothness of 2,000 to 10,000 seconds; such that the surface layer (B) has a pore size of 0.5 to 15 xcexcm; such that the surface layer (B) has a surface free energy of 33 to 65 dyn/cm; such that a content of the fine powder having a grain size of 44 xcexcm or above in the surface layer (B) is limited to 10 ppm or below; and such that a porosity of the sheet. estimated from the formula (1) described later is 5 to 60%.
The present invention also provides a method for fabricating a fusion thermal transfer recording sheet having a step for forming, on at least one side of a base layer (A) containing 40 to 85 wt % of a thermoplastic resin and 60 to 15 wt % of an inorganic or organic fine powder, a surface layer (B) containing 30 to 90 wt % of a thermoplastic resin and 70 to 10 wt % of an inorganic fine powder which has an average grain size equals to or smaller than that of the inorganic or organic fine powder contained in the base layer (A) and has a grain surface modified by hydrophilic treatment; and a step for uniaxially stretching the obtained laminate.
Preferred embodiments of the present invention are such that the uniaxial stretching is effected at a temperature lower by 5xc2x0 C. or more than the melting point of the thermoplastic resin contained in the surface layer (B) and lower by 15xc2x0 C. or more than the melting point of the thermoplastic resin contained in the base layer (A); and such that the uniaxial stretching is performed by 2 to 7.5 times in length.